Hi Hans -Can I use the disk I now use as L2ARC as a log device. You should be able to - just remove the cache device and re-add it as a log device using the zpool command. Terminology wise your actually referring to a Separate Log (SLOG), as ZFS always has an intent log (unless sync=disabled) but it rests somewhere in the regular zpool vdevs instead.
-Do I need to add another for safety? (Someone correct me if I’m wrong here) - The log device is actually already the second copy of the data that is getting the sync write. The writes also exist in RAM (like regular ZFS async writes) but just to satisfy the “it’s on safe storage” requirement, the data gets written to the log device first, as it should be faster. Once it’s written to the SLOG, the writes sit in RAM and are flushed to the regular pool vdev’s during the next write cycle. The SLOG is only ever read if someone bad happens while the writes are still sitting in RAM, such as a power failure. Although the decision is yours, in order to lose sync writes that have been acknowledged to the KVM, you would need for something bad to happen (power failure/panic etc) after the SLOG has been written to but before the writes are flushed from RAM. You would simultaneously also need your SSD to die. You can mirror your SLOGs or stripe them. -Can log devices be removed easily? In SmartOS, yes. Just use the zpool command to remove them. -SmartOS specific gotchas? With respect to KVMs, the lower the write latency, the better. Also, I would check the doco of your SSD to see whether it has super-capacitor or other power protection methods built in. These facilities ensure that in the event of power failure, any data sitting in the SSDs buffer has sufficient time to make it into persistent storage. We recently installed some SAS SSDs that, according to the spec sheets, are even faster than ZeusRAM drives. As such, I can get 476MB/sec sequential writes and 313.5MB/sec random 512KB writes out of a Windows 2k12 KVM. Thanks, Dave On 30 Apr 2015, at 6:47 am, Hans Kruse <[email protected]<mailto:[email protected]>> wrote: The question about sluggish KVM's answered my not yet asked question!!! Thanks!!! I temporally disabled sync writes on my windows 2012 KVM disk. It went from sluggish unusable to blazing fast. (installing Visual Studio). Now I want to have some guidance on installing an SSD as log device. My setup Intel 4790T 32GB Memory 2X 2Tb WD Red disks in mirror. a 120Gb SSD as L2ARC.(which I suspect does not really help). Questions: -Can I use the disk I now use as L2ARC as a log device. -Do I need to add another for safety? -Can log devices be removed easily? -SmartOS specific gotchas? smartos-discuss | Archives<https://www.listbox.com/member/archive/184463/=now> [https://www.listbox.com/images/feed-icon-10x10.jpg10f3ec5.jpg?uri=aHR0cHM6Ly93d3cubGlzdGJveC5jb20vaW1hZ2VzL2ZlZWQtaWNvbi0xMHgxMC5qcGc] <https://www.listbox.com/member/archive/rss/184463/25738179-216c4b5f> | Modify<https://www.listbox.com/member/?&;> Your Subscription [https://www.listbox.com/images/listbox-logo-small.png10f3ec5.png?uri=aHR0cHM6Ly93d3cubGlzdGJveC5jb20vaW1hZ2VzL2xpc3Rib3gtbG9nby1zbWFsbC5wbmc] <http://www.listbox.com/> ------------------------------------------- smartos-discuss Archives: https://www.listbox.com/member/archive/184463/=now RSS Feed: https://www.listbox.com/member/archive/rss/184463/25769125-55cfbc00 Modify Your Subscription: https://www.listbox.com/member/?member_id=25769125&id_secret=25769125-7688e9fb Powered by Listbox: http://www.listbox.com
